1. Field of the Invention
The present invention relates to a digital color image processor.
2. Description of the Prior Art
In a digital color image processor, black characters in a document are usually discriminated to improve reproducibility of black characters. Characters (edges) are discriminated from achromatic image portions in a document by detecting an edge portion and an achromatic portion in the document. Various techniques are proposed for discriminating an achromatic image, and they discriminate an achromatic portion according to color data, that is, red (R), green (G) and blue (B) data received by the image processor. For example, in a technique, a difference of the maximum from the minimum of R, G and B color data, that is, MAX(R, G, B)--MIN(R, G, B), is used as chroma data. A portion having a larger difference is decided to be a color image, while a portion having a lower difference is decided to be an achromatic image. On the other hand, an image scanner usually uses a reduction color sensor unit (refer to FIG. 2) which has three linear sensors in correspondence to the three colors of R, G and B. The sensors are shorter than the width of an image to be read, and a lens is provided in an optical system in the image scanner to converge light onto the reduction color sensor.
In a spectral distribution of color data having red (R), green (G) and blue (B), red light has longer wavelength components, and blue light has shorter wavelength components, as shown in FIG. 10. Further, the lens in the reduction optical system has color aberration, and when a light is incident near two ends thereof in the main scan direction, the lens converges shorter wavelength components in the incident light at the inner side (or in the side of the center of the document) and longer wavelength components at the outer side. Then, a color shift happens when a color image is read. This color shift affects the above-mentioned discrimination of an achromatic portion. It is expected that the difference, MAX(R, G, B)--MIN(R, G, B), will become minimum at a black edge portion, but the edge may erroneously be recognized as a color edge due to the color shift.